1. Field of the Invention
The invention pertains generally to the field of mitigating electromagnetic radiation generated by electronic devices, particularly devices that generate high-frequency signals. More particularly, this invention pertains to apparatus for reducing emission of electromagnetic radiation through gaps in enclosures of electronic equipment.
2. Discussion of the Related Art
In the operation of electronic devices, electromagnetic waves are generated in addition to intended electrical signals. This electromagnetic interference (or EMI) is undesirable and, if strong enough, has the ability to interfere with radio communications or the operation of nearby electronic apparatus. The Federal Communications Commission (FCC) strictly regulates the amount of electromagnetic emissions that may radiate from a product. All computer equipment sold in the United States must demonstrate compliance with FCC-mandated electromagnetic emission levels. To attenuate the amount of electromagnetic waves emitted, devices are equipped with an electromagnetic containment feature, such as a metallic enclosure, to keep the waves from exiting the device. This ensures that the electronic device will have electromagnetic compatibility with any neighboring apparatus.
Electromagnetic containment devices are typically shielded and electrically grounded. Shielding is designed to prevent both ingress and egress of electromagnetic energy. Where the shielding is incomplete due to gaps, EMI is not fully blocked and its effects not fully minimized. In the case of a metal enclosure, these gaps create discontinuities in the conductivity of the enclosure, which in turn compromises the efficiency of the ground conduction path through the enclosure. Moreover, by presenting a conductivity level at the gaps that is significantly different from that of the enclosure as a whole, the gaps can act as slot antennae, and the enclosure itself can become a secondary source of EMI. Finally, to achieve full blockage of EMI radiation over a wide bandwidth, no gaps above a specified size may exist in the shield. The higher the frequency of the EMI, the smaller the gaps must be.
In practice, computer enclosures usually have several gaps, including gaps to accommodate connectors, gaps to admit disks of various kinds, and gaps created by doors, access panels and assembly joints. Despite this, EMI requirements for most computer systems have been easily complied with in the past using relatively simple, low-cost enclosures. However, the advent of modern computer systems with chip speeds exceeding a gigahertz has made compliance with EMI requirements more difficult and rendered unacceptable the computer cabinets of the past.
Specialized EMI gaskets have been developed for use in computer gaps to provide at least a degree of EMI shielding while permitting unfettered operation of the computer enclosure doors and access panels. To shield EMI effectively, a gasket should be capable of absorbing or reflecting EMI as well as establishing a continuous conductive path across the gap in which the gasket is disposed.
Metallic gaskets manufactured from beryllium copper are widely employed for EMI shielding due to their high level of electrical conductivity. These gaskets typically take the form of fingers projecting from a strip. The fingers provide spring and wiping actions when compressed. Other types of EMI gaskets include metalized fabric, closed-cell foam sponges knitted with metallic wire mesh, silicone tubing knitted with metallic wire mesh, and strips of rolled metallic wire mesh, with or without foam or tubing inserts.
Finger-style EMI gaskets can be relatively fragile and can be damaged when not handled properly. Damage can create metal fragments, which can find their way into any of the electronic circuitry in the computer, including not only the computing circuitry, but also the circuitry of the power supply. Mechanical problems also can result, including snags, poor fit and possible injury to handlers.
Efforts to remedy these problems have centered on replacing finger-type gaskets with the soft gaskets described above. This solution has had various problems of its own, including short material life, mounting difficulties, poor EMI shielding relative to metal gaskets, flammability and compression set.
Another effort to remedy the problems with finger-style gaskets has been to use a fingerless gasket in the form of a rectangle with a raised section centered along the longitudinal axis of the rectangle, with the raised section providing the gasketing action. These gaskets have been deployed in pairs on either side of the edge of an enclosure opening and have proven to be effective in stopping leakage of EMI. However, difficulties have been encountered in fastening these gaskets where needed.
Two approaches have been tried. One has been to glue the gaskets to the enclosure with pressure-sensitive adhesive. However, the adhesive will fail if the gasket is snagged, and the adhesive may lose its grip over time. The second approach has been to use snap-on plastic clips on each side of the enclosure wall. However, these clips protrude several millimeters beyond the surface of the wall and may interfere with the gasket on the opposite side of the wall.